50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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The North Island Fault System (NIFS) is the<br />
principal strike-slip fault system of the Hikurangi<br />
margin, along which the Pacific Plate is being<br />
subducted beneath the overriding Australian Plate.<br />
The NIFS accommodates up to 50% of the marginparallel<br />
relative plate motion and extends for<br />
approximately 450 km, from the Wellington region<br />
in the south, to the Bay of Plenty coastline in the<br />
north. For its northern c.120 km, the NIFS<br />
comprises three main sub-parallel strands<br />
(Waiohau, Whakatane & Waimana faults).<br />
Northwards along these faults, late Quaternary rates<br />
of strike-slip decrease as rates of normal-slip<br />
increase, a transition across which horizontal to<br />
vertical slip-ratios decrease from 5-10:1 to 0.6:1<br />
(Mouslopoulou et al., 2004). In this talk we will<br />
identify the position of this kinematic transition on<br />
each of the main faults, and assess whether it may<br />
have functioned as a mechanical barrier to rupture<br />
propagation during the history of paleoearthquakes<br />
known for this region. We use fault-trench log data<br />
from 15 trenches and >50 measurements of offset<br />
landforms. The ages of key stratigraphic markers<br />
are constrained by tephra glass chemistry and 14 C<br />
dating. Preliminary results suggest that the<br />
Whakatane Fault last ruptured since 0.8 kyr BP,<br />
while the Waimana and Waiohau faults last<br />
ruptured 1.8-2.8 and 1.8-5.6 kyr ago, respectively.<br />
Within c. 40 km of the Bay of Plenty coastline,<br />
earthquake recurrence intervals average about 2-<br />
2.5, 3 and 4.5 kyr on the Whakatane, Waimana, and<br />
Waiohau faults, respectively. On individual faults,<br />
the timing and number of earthquakes during the<br />
Holocene appears to have varied along strike across<br />
the kinematic transition zone. On the<br />
Ruahine/Waiohau Fault, for example, the strike-slip<br />
Ruahine Fault generated 6 Holocene earthquakes,<br />
while to the north in the Galatea Basin, where the<br />
fault is predominantly normal-slip, only 1-2<br />
Holocene earthquakes have been recorded<br />
(Beanland, 1995; Hansen, 1997). A decrease in the<br />
number of Holocene earthquakes also appears to<br />
occur northwards along the Whakatane Fault,<br />
across the kinematic transition zone (between<br />
Ruatahuna to Ruatoki North). Although not<br />
conclusive, the data suggest that some<br />
paleoearthquakes on the Whakatane and Waiohau<br />
faults terminated within the kinematic transition<br />
zone, which functioned as a mechanical barrier to<br />
propagation of large ruptures. The along-strike<br />
change in fault kinematics, defined by a 60-70°<br />
northward steepening in the pitch of the slip vector<br />
on the faults, may therefore separate fault segments<br />
which ruptured during strike slip and normal slip<br />
dominated earthquakes.<br />
Beanland, S. 1995. PhD Thesis, Victoria University of<br />
Wellington, New Zealand.<br />
Hansen J., 1998. PhD Thesis. Massey University, New<br />
Zealand.<br />
Mouslopoulou et al., 2004. Geol. Soc. NZ Miscell. Pub.<br />
117A<br />
GSNZ VOICES FROM THE PAST<br />
ORAL<br />
Simon Nathan<br />
Te Ara: Encyclopedia of New Zealand<br />
Ministry for Culture & Heritage, P.O. Box 5364,<br />
Wellington<br />
(simon.nathan*mch.govt.nz)<br />
In 1955 the geological community in New Zealand<br />
was rather different from today. The four university<br />
departments were small, and the majority of<br />
geologists (almost entirely male) worked for the<br />
Geological Survey. The lack of a mining industry<br />
meant that there were few job openings for<br />
graduates. Geology was almost entirely based on<br />
surface outcrops, and there was little knowledge of<br />
offshore geology or the extent of major<br />
hydrocarbon-bearing sedimentary basins.<br />
This talk focuses on some of the individuals who<br />
founded the Geological Society of New Zealand in<br />
1955, and will be illustrated by a selection of voices<br />
from the past.<br />
ORAL<br />
TWIN PEAKS: THE PRE-MANGERE<br />
FORMATION SEQUENCE ON MANGERE<br />
ISLAND, CHATHAM ISLANDS<br />
V.E. Neall, G.Davies&R.C.Wallace<br />
Institute of Natural Resources, Massey University,<br />
Palmerston North<br />
(V.E.Neall*massey.ac.nz)<br />
During early <strong>2005</strong>, whilst studies of the Mangere<br />
Formation on Mangere Island were in progress as<br />
part of the CHEARS Marsden project, an<br />
opportunity arose to investigate the pre-Mangere<br />
Formation sequence. This paper re-examines the<br />
pre-Mangere Formation sequence on Mangere<br />
Island and proposes a new Twin Peaks model.<br />
Rangiauria Breccia is mapped as the major volcanic<br />
unit forming the edifice of Mangere and Little<br />
Mangere Islands. Two distinct lithofacies were<br />
originally recognised by Hay et al. (1970), but<br />
subsequently amalgamated by Campbell et al.<br />
(1988). The highly lithified limburgitic breccia<br />
lithofacies forms the high bluffs of northern<br />
Mangere Island and Little Mangere Island. In<br />
contrast, a weakly consolidated limburgitic breccia<br />
lithofacies forms the linear southern flank of<br />
Mangere Island. Orientation of bedding within<br />
these lithofacies points to more than 6 centres of<br />
50 th <strong>Kaikoura</strong>05 -56- <strong>Kaikoura</strong> <strong>2005</strong>